In a backbone optical communication system, a Wavelength Division Multiplexing (WDM) system is employed to increase transmission capacity and transmission speed.
In the above optical transmission system using the WDM system, optical amplification relay apparatuses each having an optical relay function, and optical transmission equipment having an add/drop function of optical signals in addition to the optical relay function are connected in cascade through an optical transmission line fiber.
The above optical transmission equipment includes a light receiving apparatus which receives an optical signal correspondingly to each wavelength of the optical signals transmitted by wavelength multiplexing. Further, in the light receiving apparatus, the presence or absence of the optical signal is detected, which is then reported to the optical transmission equipment on the subsequent stage, as information.
In such the light receiving apparatus, a WDM preamplifier is frequently used on the input side. The transmitted WDM optical signal is amplified by the WDM preamplifier, which is then separated channel-by-channel (wavelength-by-wavelength) in a demultiplexer.
Also, as the WDM preamplifier, an optical fiber amplifier having an optical fiber, in which a rare-earth element, for example, Erbium is doped on a core portion, is used. To such the optical fiber amplifier, a pumping light is supplied, and an input light is amplified by means of stimulated emission.
Further, at the time of the optical amplification, a spontaneous emission (ASE: Amplified Spontaneous Emission) light is output from the optical fiber amplifier, in addition to the amplified input light. By the accumulation of the above ASE light output from the optical fiber amplifier and the spontaneous emission light, generated from a multiplicity of optical fiber amplifiers for use for long distance transmission, a large noise light is produced after the transmission.
On the other hand, as the transmission distance becomes longer, a level input to an amplifier for a single wave after the wavelength separation by the demultiplexer becomes lower. Therefore, when the above single-wave amplifier performs ALC (Automatic Level Control) operation, the level of the output ASE light becomes high because of an increased gain.
As such, when there is no optical signal in the receiving apparatus, it is necessary to report the above state (a light input break) to the transmission equipment on the subsequent stage, and however, if the level of the ASE light is in a state higher than the level of the received signal light, it is not possible to discriminate the presence or absence of the signal only by the light level.
Therefore, in order to discriminate whether the light being input to the light receiving apparatus is an optical signal or an ASE light, it is considered to decide the presence or absence of an extracted clock of the optical signal using a clock extraction function provided in the light receiving apparatus.
However, as a bitrate becomes higher, a dispersion compensator is inserted before the light receiving apparatus, because of the necessity to compensate residual wavelength dispersion by the fiber transmission line.
As conventional techniques, an invention is described in the first patent document of Japanese Laid-open Patent Publication No. Hei-9-326755, which indicates that a clock signal is used for control. According to the invention, an light receiving apparatus is configured of an optical amplifier, a variable dispersion equivalent circuit and a clock detection circuit in the above order, so as to equalize wavelength dispersion according to a clock signal level.
Also, another invention is described in the second patent document of the Japanese Laid-open Patent Publication No. Hei-11-88261, which indicates that a compensation amount of a variable dispersion compensator is controlled so that a clock frequency component becomes maximum or minimum.
However, in any of the above first and second patent documents, there has been no technical disclosure in regard to the decision of the presence or absence of an optical signal.